Ethylbenzene (EB), paraxylene (PX), orthoxylene (OX), and metaxylene (MX) are often present together in C8 aromatic product streams from chemical plants and oil refineries. High purity EB is an important raw material for the production of styrene; however, for a variety of reasons all high purity EB feedstocks used in styrene production are produced by alkylation of benzene with ethylene, rather than by recovery from a C8 aromatics stream. Of the three xylene isomers, PX has the largest commercial market and is used primarily for manufacturing terephthalic acid and terephthalate esters for use in the production of various polymers such as poly(ethylene terephthalate), poly(propylene terephthalate), and poly(butene terephthalate). While OX and MX are useful as solvents and raw materials for making products such as phthalic anhydride and isophthalic acid, market demand for OX and MX and their downstream derivatives is much smaller than that for PX.
Given the higher demand for PX as compared with its other isomers, there is significant commercial interest in maximizing PX production from any given source of C8 aromatic materials. However, there are a number of major technical challenges to be overcome in achieving this goal of maximizing PX yield. For example, the four C8 aromatic compounds, particularly the three xylene isomers, are usually present in concentrations dictated by the thermodynamics of production of the C8 aromatic stream in a particular plant or refinery. As a result, the PX production is limited, at most, to the amount originally present in the C8 aromatic stream unless additional processing steps are used to increase the amount of PX and/or to improve the PX recovery efficiency. A variety of methods are known to increase the concentration of PX in a C8 aromatics stream. These methods normally involve cycling the stream between a separation step, in which at least part of the PX is recovered to produce a PX-depleted stream, and a xylene isomerization step, in which the PX content of the PX-depleted stream is returned back towards equilibrium concentration.
In a typical aromatics plant, such as that shown in FIG. 1, liquid feed, typically a C8+ aromatic feedstream which has previously been processed by known methods to remove C7− species (particularly benzene and toluene), is fed by conduit 1 to xylenes re-run 3, an apparatus per se well known in the art. The xylenes re-run (or more simply a fractionation column) 3 vaporizes the feed and separates the C8 aromatics into an overhead mixture 5 of xylenes and EB, and a bottom product 61 comprising C9+ aromatics. The overhead mixture typically has a composition of about 40-50% MX, 15-25% PX, 15-25% OX, and 10-20% EB. Unless otherwise noted herein, percentages are % weight. The overhead is then condensed in condenser 7, an apparatus also per se well-known in the art, and becomes the feed for the PX recovery unit 15, via conduit 9 and 13, a portion of the condensed overhead may be returned to re-run 3 as reflux via conduits 9 and 11.
The PX recovery unit 15 may employ crystallization technology, adsorption technology, or membrane separation technology, each per se well known in the art. These technologies separate PX from its isomers and are capable of producing high purity PX up to 99.9%, which is taken from unit 15 via conduit 17. Shown in FIG. 1 is the case where unit 15 is an adsorptive separation unit, such as a Parex™ or Eluxyl™ unit, in which case typically the extract 17, which comprises a desorbent, such as paradiethylbenzene (PDEB), needs to be separated, such as by distillation, from the desired extract PX in distillation column 19, which generates an overhead 23 that is condensed in condenser 25 to yield a liquid stream 27, which is a high purity PX stream. This stream 27 may be taken off via conduit 31 and optionally a portion may be returned to column 19 as reflux via conduit 29. The desorbent is returned to the PX recovery system 15 via conduit 21. Raffinate from the recovery system 15, comprising MX, OX, EB, and some PX, is removed via conduit 65 and sent to unit 37, discussed below. Note: a portion of raffinate in 65 may be recovered and marketed as low-value solvent xylene.
The raffinate 65, which comprises mainly MX, OX, EB, and desorbent is sent to fractionation column 37, generating overhead 33 and bottoms 63. Overhead 33 contains MX, OX and EB, which is condensed in condenser 32 and sent via conduit 35 and then 41 to isomerization unit 43, discussed in more detail below. A portion may be returned to fractionator 37 via conduit 35 and then 39 as reflux. The desorbent in the bottoms product is returned to 15. Note that as used herein the term “raffinate” is used to mean the portion recovered from the PX recovery unit 15, whether the technology used is adsorptive separation, crystallization, or membrane, and then is sent to the isomerization unit 43, conventionally a vapor phase isomerization unit, which uses technology also per se well-known.
A stream consisting essentially of MX, OX, and EB is sent to isomerization unit 43, an apparatus per se known in the art, to isomerize the MX and OX and optionally EB to PX. Isomerization unit 43 may be a vapor phase or liquid phase isomerization unit. Conventionally there are one or more heat exchangers or furnaces associated with the system shown in FIG. 1 between the PX recovery unit 15 and the isomerization unit that are not shown for convenience of view. Likewise, hydrogen separators and hydrogen compressors are also not shown for convenience of view. These and other features, such as valves and the like, would be apparent to one of ordinary skill in the art.
The product of the isomerization unit 43 is sent via conduit 51 to the C7− distillation tower 53, which separates the product of isomerization into a bottom stream 59 comprising equilibrium xylenes and the overhead 47, comprising C7− aromatics, e.g., benzene and toluene. The overhead product is condensed in condenser 45 and then the distribution of liquid product via conduit 49 may be apportioned as desired between conduit 57 and conduit 55, the former of which may be disposed of in numerous ways which would be well-known per se in the art, and the latter conduit returning C7− aromatics as reflux to tower 53. The bottoms product 59 of distillation tower 53 is then sent to xylenes re-run 3, either merging with feed 1 as shown in the figure, or it may be introduced by a separate inlet (not shown).
Vapor phase isomerization of the raffinate's EB is generally needed to limit EB buildup in the recycle stream 59. However, vapor phase isomerization has many disadvantages, including high energy consumption, costly and complex process equipment, and high xylenes loss due to conversion of the xylenes in the raffinate into undesirable products such as light gases and heavy aromatics, e.g., by one or more side-reactions such as one or more of cracking, transalkylation, or disproportionation. Attempts to overcome these disadvantages include reducing the quantity of raffinate going to the vapor phase isomerization, e.g., removing EB from the raffinate by using chromatographic EB separation in the PX recovery unit, as disclosed in U.S. Pat. No. 7,915,471 and U.S Patent Publication No. 2015/0266794. The separated EB is isomerized in a vapor phase isomerization stage, with the remainder of the raffinate being isomerized in a liquid phase isomerization stage. The liquid phase isomerization stage is operated under conditions which lessen undesired cracking, transalkylation, and disproportionation side-reactions. Isomerates from the vapor phase and liquid phase isomerization stages are then combined and recycled to the xylene re-run column.
Even when EB is separated for vapor phase isomerization, with the remainder of the PX-depleted raffinate subjected to liquid phase isomerization, the vapor phase isomerization stage contributes to xylene loop inefficiencies. Some of these inefficiencies result from one or more of (i) the need to vaporize the separated EB and then re-condense the vapor phase isomerate for combining with the isomerate derived from the liquid phase isomerization stage, (ii) the need to separate unreacted molecular hydrogen vapor for re-use as an isomerization treat gas, and (iii) the need for removing non-aromatics formed during isomerization. Consequently, it is desired to further lessen or even eliminate the need for vapor phase isomerization.